1. Field of the Invention
This invention relates to a direct-current motor having an armature including a yoke, a plurality of teeth integrally extending from the yoke in a circular disposition so as to define a plurality of slots, and coils wound on the respective teeth.
2. Description of the Prior Art
An armature for a conventional direct-current (DC) motor, for example, a stator for a three-phase brushless DC motor of the outer rotor type, has a stator core including a yoke and a number of teeth integrally extending from the yoke in a circular disposition so as to define a number of slots and provided with respective coils wound thereon. A distal end opening of each slot is half-closed so that each slot is formed into a semienclosed slot. Consequently, a variation of permeance between the stator core and a rotor in a direction of rotation of the rotor or in the circumferential direction is reduced such that a cogging torque which is substantially a torque variation can be reduced.
In another conventional DC motor, the teeth have two alternately different circumferential widths at the respective distal ends thereof. In this construction, the semienclosed slots have alternately different circumferential pitches at the centers of the openings thereof. The randomicity of the circumferential pitches reduces the variations of permeance. This effectively reduces the cogging torque.
An automatic winding machine is usually used to wind coils on the respective teeth of the stator core for the DC motor. Guide members referred to as "formers" are reciprocally moved lengthwise with respect to each tooth so that a wire, namely, a magnet wire is guided to be moved for every one turn pitch. Thus, each coil is formed into a predetermined shape.